1. Field of the Invention
The present invention relates to a stepping motor controller, a scanning apparatus and an image forming apparatus including the stepping motor controller, such as a copy machine, a printer, or a facsimile.
It is known that “stepping motor” is also called “pulse motor”, “step motor”, or “stepper motor”. In the present specification, the term “stepping motor” is used to represent any rotary motor or liner motor that generates electrical power by switching magnetic excitation states of magnetic poles therein, just as an electric motor does, which is given one of the names “stepping motor”, “pulse motor”, “step motor”, or “stepper motor”.
2. Description of the Related Art
Generally, an electric motor, for example, a stepping motor, generates electrical power by switching magnetic excitation states of magnetic poles therein. A basic characteristic of the stepping motor is that it has a portion movable due to an attractive force or a repulsive force resulting from a magnetic force, wherein the movable portion is moved by switching magnetic excitation states of the magnetic poles therein, and thereby, the position of the movable portion can be controlled with high precision.
The magnetic poles of the stepping motor are formed by the movable portion and stationary coils, and the movable portion is made up of permanent magnets or magnetic material. Generally, by switching the magnetic excitation states of the magnetic poles, an attractive force and a repulsive force are generated to drive the movable portion, and by fixing the magnetic excitation states of the magnetic poles, the movable portion is maintained at rest.
The structure of the stepping motor can be classified into a VR type (variable magnetic resistant), a PM type (permanent magnet), and a hybrid (HB) type that combines the above two. There have been manufactured stepping motors having two-phase magnetic poles, three-phase magnetic poles, four-phase magnetic poles, and five-phase magnetic poles.
A VR type stepping motor is driven by the attractive force between rotors that form teeth of the motor and stators of the magnetic poles. A PM type stepping motor is driven by the attractive force and repulsive force between a rotor formed by a permanent magnet having alternatively arranged N poles and S poles and stators of the magnetic poles. The HB type stepping motor has a structure combining those of the VR type and PM type.
In a range in which the rotating speed of the stepping motor does not change even though the load of an object driven by the motor changes, the inductance of the excitation coil does not change; therefore, in this case, the power consumption is constant when the rotating speed of the motor is constant or when the motor is at rest. In other words, when the load is light, power is wasted under the condition that the rotating speed of the motor is constant or the motor is at rest.
However, maintaining a constant rotating speed of the stepping motor means that the aforementioned movable portion should operate in a predetermined manner when switching the magnetic poles, and a magnetic force is required for driving the movable portion to correctly move among the magnetic poles that are being switched. After the required magnetic force is determined, using this magnetic force is sufficient if the load is constant. However, if the load changes and if the change is not clearly determined, a magnetic force corresponding to the maximum load variation has to be provided, in other words, the maximum excitation current has to be supplied.
By raising the excitation current, the driving torque rises, and the probability of the stepping motor being out-of-step becomes low. However, if an unnecessarily large current is supplied, power consumption rises. In addition, there arise problems such as vibration, noise, and heat of the motor. Therefore, it is desirable that the current be set to an appropriate value.
The following references show the related art.
Japanese Laid Open Patent Application No. 5-56690 discloses a method for setting the excitation current, including steps of measuring the driving current that does not cause out-of-step by gradually increasing the driving current of a stepping motor, and determining the driving current to be set for the stepping motor based on this measurement. In addition, during the operation, the actual current of the stepping motor is measured, and the actual current is adjusted to be the same as an object current value by feedback control.
Japanese Laid Open Patent Application No. 11-215890 discloses a motor controller that, by using a CPU, measures the driving current when out-of-step occurs by gradually decreasing the driving current of a stepping motor, and determines the usual driving current based on this measurement.
Japanese Laid Open Patent Application No. 6-43050 discloses a method of measuring an actual load torque of a stepping motor coupled with load machinery.
Japanese Laid Open Patent Application No. 2000-23496 discloses a step motor controller equipped with a computer for measuring out-of-step.
When setting a constant excitation current flowing in magnetic poles, which rotate at a constant speed in a driving system, the reliability of the system in avoiding the out-of-step condition is not certain. Generally, this reliability is estimated by multiplying a coefficient greater than one by the out-of-step current while decreasing the excitation current. This estimation is good if the coefficient is empirically determined.
In the aforementioned case of a stepping motor driven by a constant current, the actual driving current is calculated or measured beforehand; taking into consideration the fluctuation or time variation of the load, the value of the driving current is set to a value capable of driving an expected largest load so as to enable driving having a sufficiently large safety margin. Adversely, power consumption of the motor rises, and much heat is generated.